What does the mouse-binding "in-between move" do?
This is an event used internally for painting and sculpting tools, it is not useful in a keymap. It can be used when writing certain tools in Python (typically modal operators).

What this event does is give you higher resolution mouse positions in between the regular mouse move events. For most tools only the latest mouse position matters, and processing the positions in between would just be wasting time. But for some tools like painting and sculpting it's useful to known the full path that the mouse traveled to get more accurate paint strokes.